To improve the synergistic explosion suppression and flame retardant efficiency of inert gas-water mist，based on spectral experiments，theoretical analysis and numerical simulation methods such as Fluent and CHEMKIN-PRO were used to study the changes in elementary reactions and typical free radical (H· and ·OH) molar fractions during the explosion suppression and flame retardant process of N₂ water mist at different injection positions and pressures. The results showed that staggered injection of N₂ and fine water mist had better synergistic explosion suppression and flame retardant effect than convection and parallel injections. For case of simulated pipeline，good explosion suppression and flame retardant effect were found only when the spray pressure of N₂ and water mist was 4.5 and 2 MPa，respectively. Furthermore，the maximum value of mole fraction for H· and ·OH was 0.006 4 and 0.006 9，respectively. The spray pressure can be applied to the actual methane explosion fire area based on scale effect. Moreover，the H· and ·OH mole fractions can be used as monitoring parameters during the operation of the current automatic fire extinguishing system in the fire protection area，and their changes can be used as a reference for fire extinguishing process.